1. Field of the Invention
The invention relates to an electric power transmission device, and in particular to an electric power transmission device that contactlessly or wirelessly transmits electric power to an electric power receiving device.
2. Description of Related Art
In a system of contactlessly or wirelessly transmitting electric power from an electric power transmission device to an electric power receiving device, it has been proposed, as a known technology of this type, to control the power-supply frequency of the electric power transmission device based on normalized power-transmission current (see, for example, Japanese Patent Application Publication No. 2014-103754 (JP 2014-103754 A)). The normalized power-transmission current is defined as the ratio of second power-transmission current to the maximum value of first power-transmission current. The first power-transmission current is defined as power-transmission current of the electric power transmission device measured when the electric power transmission device and the electric power receiving device are in a non-coupled state, and the second power-transmission current is defined as power-transmission current of the electric power transmission device measured when the electric power transmission device and the electric power receiving device are in an induction-coupled state. When the normalized power-transmission current is equal to or greater than ½, the power-supply frequency is set to the resonance frequency. When the normalized power-transmission current is less than ½, the power-supply frequency is controlled to be varied so that the normalized power-transmission current becomes equal to ½. With the power-supply frequency thus controlled, it is possible to increase received electric power, and maximize the electric power efficiency, only through control of the power-supply frequency of the electric power transmission device.
An electric power transmission device of a contactless electric power transmission system often includes an inverter that is driven under pulse width modulation (PWM) control so as to adjust the frequency and voltage of AC power to be transmitted. In this case, the inverter generally consists of four switching devices Q91-Q94, and four diodes D91-D94 connected in inverse-parallel with the switching devices Q91-Q94, respectively, as shown in FIG. 8. The switching devices Q91-Q94 are grouped into two pairs, each having two devices serving as a source and a sink and located between a positive bus and a negative bus, and opposite terminals of a power transmission coil are connected to respective connecting points of the paired switching devices.
In the electric power transmission device including the inverter as described above, the phase of electric current may lead that of alternating voltage developed under the PWM control. FIG. 9 shows one example of the relationship among the ON/OFF states of the switching devices Q91-Q94 and the output voltage and current of the inverter. In a section labelled as “INVERTER OUTPUT VOLTAGE, CURRENT” in FIG. 9, the solid stepped line represents output voltage, and the solid sine curve represents current at the time when the current phase leads the voltage phase. Considering that the switching device Q91 is now shifting from the OFF state to the ON state, the inverter output voltage is equal to zero, but the current, whose phase leads the voltage phase, assumes a positive value, at time T1 when the switching device Q91 is in the OFF state. At this time, the current flows from a lower power line on the power transmission coil side, to the switching device Q94 that is in the ON state, the switching device Q93 that is in the ON state and diode D93, and an upper power line on the power transmission coil side, in the order of description, as shown in FIG. 10A. At time T2 immediately after the switching device Q91 is turned on, the inverter output voltage assumes a positive value, and the current is kept being a positive value. At this time, the current flows from the positive bus (upper bus) to the upper power line on the power transmission coil side via the switching device Q91 that is in the ON state, and flows from the lower power line on the power transmission coil side to the negative bus (lower bus) via the switching device Q94 that is in the ON state, as shown in FIG. 10B. A forward bias is applied to the diode D93 at time T1 when the switching device Q91 is in the OFF state, and a reverse bias is applied to the diode D93 at time T2 immediately after the switching device Q91 is turned on. Therefore, recovery current flows through the diode D93 as indicated by the thick arrow in FIG. 10B, due to a recovery characteristic of the diode. Since the recovery current results in short-circuit current, it may cause abnormal heating or failure of the electric power transmission device.